A cardigan worn closed stops being a layer and becomes the top, which changes what you should be looking for. Fit matters at the shoulder and the bust rather than the length, and the button placket becomes the front detail of the outfit, so its spacing and the size of the buttons are suddenly visible. A fine gauge works best, because a chunky knit worn closed adds bulk exactly where a top should be clean. It also means the thing underneath is structural rather than decorative, so a smooth base layer that does not show through is worth more than another knit.

The failure mode is gapping between buttons across the bust, which is a fit issue rather than a size issue: a slightly looser cardigan in a finer yarn gaps less than a tight one in a heavier yarn. Neckline is the second decision. A crew neck buttoned to the top reads neat and slightly formal. Leaving the top two buttons open makes a V and softens it. A deep V-front cardigan worn closed needs something underneath, which is fine as long as that layer is deliberate. Sleeve length is worth checking too, since a cardigan sleeve cut for layering is often longer than a top sleeve should be.

We came to this because we own more cardigans than tops, and it turns out most of them work better closed than open. The one rule we would insist on is fine gauge. A heavy cardigan buttoned up looks like a jumper that has been put on backwards, and no styling recovers it. We would also say to size for the bust and accept a slightly loose shoulder, since gapping is the thing that will stop you wearing it. If you take one thing away, it is that a closed cardigan is a top, so judge it on how a top should fit rather than how a layer should. If you want to keep browsing, I also put together The Cardigan as a Top and Top-Handle Bags.